Preparation of substituted isoserine esters using beta-lactams and metal or ammonium alkoxides

ABSTRACT

A process for preparing N-acyl, N-sulfonyl and N-phosphoryl substituted isoserine esters in which a metal or an ammonium alkoxide is reacted with a β-lactam.

REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. Ser. No. 10/194,343,filed Jul. 12, 2002, which is a continuation of U.S. Ser. No.09/516,870, filed on Mar. 2, 2000, which is a continuation of U.S. Ser.No. 08/938,075, filed on Sep. 26, 1997, now U.S. Pat. No. 6,124,481,which is a division of U.S. Ser. No. 08/476,357, filed on Jun. 7, 1995,now U.S. Pat. No. 5,717,115, which is a division of U.S. Ser. No.08/034,247, filed on Mar. 22, 1993, now U.S. Pat. No. 5,430,160.

BACKGROUND OF THE INVENTION

[0002] Three esters of N-acyl phenyl isoserine, taxol, taxotere andcephalomannine have been found to possess significant properties asantitumer agents. This application describes a process for thepreparation of N-acyl, N-sulfonyl and N-phosphoryl substituted isoserineesters, in general and to a semi-synthesis for the preparation of taxanederivatives such as taxol, taxotere and other biologically activederivatives involving the use of metal alkoxides and β-lactams, inparticular.

[0003] The taxane family of terpenes, of which taxol is a member, hasattracted considerable interest in both the biological and chemicalarts. Taxol is a promising cancer chemotherapeutic agent with a broadspectrum of antileukemic and tumor-inhibiting activity. Taxol has thefollowing structure:

[0004] wherein Ac is acetyl. Because of this promising activity, taxolis currently undergoing clinical trials in both France and the UnitedStates.

[0005] The supply of taxol for these clinical trials is presently beingprovided by the bark from Taxus brevifollia (Western Yew). However,taxol is found only in minute quantities in the bark of these slowgrowing evergreens, causing considerable concern that the limited supplyof taxol will not meet the demand. Consequently, chemists in recentyears have expended their energies in trying to find a viable syntheticroute for the preparation of taxol. So far, the results have not beenentirely satisfactory.

[0006] One synthetic route that has been proposed is directed to thesynthesis of the tetracyclic taxane nucleus from commodity chemicals. Asynthesis of the taxol congener taxusin has been reported by Holton, etal. in JACS 110, 6558 (1988). Despite the progress made in thisapproach, the final total synthesis of taxol is, nevertheless, likely tobe a multi-step, tedious, and costly process.

[0007] A semi-synthetic approach to the preparation of taxol has beendescribed by Greene, et al. in JACS 110, 5917 (1988), and involves theuse of a congener of taxol, 10-deacetyl baccatin III which has thestructure of formula II shown below:

[0008] 10-deacetyl baccatin III is more readily available than taxolsince it can be obtained from the needles of Taxus baccata. According tothe method of Greene et al., 10-deacetyl baccatin III is converted totaxol by attachment of the C-10 acetyl group and by attachment of theC-13 β-amido ester side chain through the esterification of the C-13alcohol with a β-amido carboxylic acid unit. Although this approachrequires relatively few steps, the synthesis of the β-amido carboxylicacid unit is a multi-step process which proceeds in low yield, and thecoupling reaction is tedious and also proceeds in low yield. However,this coupling reaction is a key step which is required in everycontemplated synthesis of taxol or biologically active derivative oftaxol, since it has been shown by Wani, et al. in JACS 93, 2325 (1971)that the presence of the β-amido ester side chain at C13 is required foranti-tumor activity.

[0009] More recently, it has been reported in Colin et al. U.S. Pat. No.4,814,470 that taxanes corresponding to the following formula III, havean activity significantly greater than that of taxol (I).

[0010] R′ represents hydrogen or acetyl and one of R″ and R′″ representshydroxy and the other represents tert-butoxy-carbonylamino and theirstereoisomeric forms, and mixtures thereof.

[0011] According to Colin et al., U.S. Pat. No. 4,418,470, the productsof general formula (III) are obtained by the action of the sodium saltof tert-butyl N-chlorocarbamate on a product of general formula:

[0012] in which R′ denotes an acetyl or 2,2,2-trichloroethoxy-carbonylradical, followed by the replacement of the2,2,2-trichloroethoxycarbonyl group or groups by hydrogen. It isreported by Denis et al. in U.S. Pat. No. 4,924,011, however, that thisprocess leads to a mixture of isomers which has to be separated and, asa result, not all the baccatin III or 10-deactylbaccatin III employedfor the preparation of the product of general formula (IV) can beconverted to a product of general formula (III).

[0013] In an effort to improve upon the Colin et al. process, Denis etal. disclose a different process for preparing derivatives of baccatinIII or of 10-deactylbaccatin III of general formula

[0014] in which R′ denotes hydrogen or acetyl wherein an acid of generalformula:

[0015] in which R₁ is a hydroxy-protecting group, is condensed with ataxane derivative of general formula:

[0016] in which R₂ is an acetyl hydroxy-protecting group and R₃ is ahydroxy-protecting group, and the protecting groups R₁, R₃ and, whereappropriate, R₂ are then replaced by hydrogen. However, this methodemploys relatively harsh conditions, proceeds with poor conversion, andprovides less than optimal yields.

[0017] A major difficulty remaining in the synthesis of taxol and otherpotential anti-tumor agents is the lack of a readily available methodfor easy attachment, to the C-13 oxygen, of the chemical unit whichprovides the β-amido ester side chain. Development of such a process forits attachment in high yield would facilitate the synthesis of taxol aswell as related anti-tumor agents having a modified set of nuclearsubstituents or a modified C-13 side chain. This need has been fulfilledby the discovery of a new, efficient process for attachment, to the C-13oxygen, of the chemical unit which provides the β-amido ester sidechain.

[0018] Another major difficulty encountered in the synthesis of taxol isthat known processes for the attachment of the β-amido ester side chainat C-13 are generally not sufficiently diastereoselective. Therefore,the side chain precursor must be prepared in optically active form toobtain the desired diastereomer during attachment. The process of thisinvention, however, is highly diastereoselective, thus permitting theuse of a racemic mixture of side chain precursor, eliminating the needfor the expensive, time-consuming process of separating the precursorinto its respective enantiomeric forms. The reaction additionallyproceeds at a faster rate than previous processes, thus permitting theuse of less side-chain precursor than has been required by such previousprocesses.

SUMMARY OF THE INVENTION

[0019] Among the objects of the present invention, therefore, is theprovision of a process for the preparation of N-acyl, N-sulfonyl andN-phosphoryl esters of isoserine; the provision of a side chainprecursor for the synthesis of taxane derivatives; the provision of aprocess for the attachment of the side chain precursor in relativelyhigh yield to provide an intermediate which is readily converted to thedesired taxane derivative; and the provision of such a process which ishighly diastereoselective.

[0020] In accordance with the present invention, a process is providedfor the preparation of isoserine esters having the formula

[0021] comprising reacting a β-lactam with an alkoxide, the β-lactamhaving the formula

[0022] and the alkoxide having the formula

MOCE₁E₂E₃

[0023] wherein

[0024] R₁ is —OR₆, —SR₇, or —NR₈R₉;

[0025] R₂ is hydrogen, alkyl, alkenyl, alkynyl, aryl, or heteroaryl;

[0026] R₃ and R₄ are independently hydrogen, alkyl, alkenyl, alkynyl,aryl, heteroaryl, acyl or heterosubstituted alkyl, alkenyl, alkynyl,aryl or heteroaryl, provided, however, that R₃ and R₄ are not both acyl;

[0027] R₅ is —COR₁₀, —COOR₁₀, —COSR₁₀, —CONR₈R₁₀, or —SO₂R₁₁,

[0028] R₆ is hydrogen, alkyl, alkenyl, alkynyl, aryl, heteroaryl,hydroxy protecting group, or a functional group which increases thewater solubility of the taxane derivative,

[0029] R₇ is alkyl, alkenyl, alkynyl, aryl, heteroaryl, or sulfhydrylprotecting group,

[0030] R₈ is hydrogen, alkyl, alkenyl, alkynyl, aryl, heteroaryl, orheterosubstituted alkyl, alkenyl, alkynyl, aryl or heteroaryl;

[0031] R₉ is an amino protecting group;

[0032] R₁₀ is alkyl, alkenyl, alkynyl, aryl, heteroaryl, orheterosubstituted alkyl, alkenyl alkynyl, aryl or heteroaryl;

[0033] R₁₁ is alkyl, alkenyl, alkynyl, aryl, heteroaryl, —OR₁₀, or—NR₈R₁₄;

[0034] R₁₄ is hydrogen, alkyl, alkenyl, alkynyl, aryl, or heteroaryl;

[0035] E₁, E₂ and E₃ are independently hydrogen, hydrocarbon or cyclic,provided, at least one of E₁, E₂ and E₃ is other than hydrogen; and

[0036] M comprises ammonium or is a metal.

[0037] In accordance with another aspect of the present invention, thealkoxide and β-lactam are selected so as to provide a process forpreparing taxol, taxotere and other biologically active taxanederivatives having the following structural formula:

[0038] wherein

[0039] R₁-R₁₄ are as previously defined,

[0040] R₁₅ is hydrogen or together with R₁₆ forms an oxo,

[0041] R₁₆ is hydrogen, —OCOR₂₉, hydroxy, or protected hydroxy, ortogether with R₁₅ forms an oxo;

[0042] R₁₇ is hydrogen or together with R₁₈ forms an oxo,

[0043] R₁₈ is hydrogen, hydroxy, protected hydroxy, acyloxy, or togetherwith R₁₇ forms an oxo;

[0044] R₁₉ is hydrogen or together with R₂₀ forms an oxo,

[0045] R₂₀ is hydrogen, halogen, protected hydroxy, —OR₂₈, or togetherwith R₁₉ forms an oxo;

[0046] R₂₁ is hydrogen or together with R₂₂ forms an oxo,

[0047] R₂₂ is hydrogen, hydroxy, protected hydroxy, acyloxy, togetherwith R₂₁ forms an oxo, or together with R₂₃ and the carbon atoms towhich they are attached form an oxetane ring;

[0048] R₂₃ is hydrogen, together with R₂₄ forms an oxo, or together withR₂₂ and the carbon atoms to which they are attached form an oxetanering;

[0049] R₂₄ is hydrogen, alkyl, alkenyl, alkynyl, aryl, heteroaryl,cyano, hydroxy, —OCOR₃₀, or together with R₂₃ forms an oxo, oxirane ormethylene;

[0050] R₂₅ is hydrogen, hydroxy, or —OCOR₃₁;

[0051] R₂₆ is hydrogen or taken together with R₂₅ forms an oxo;

[0052] R₂₇ is hydrogen, hydroxy, protected hydroxy;

[0053] R₂₈ is hydrogen, acyl, hydroxy protecting group or a functionalgroup which increases the solubility of the taxane derivative; and

[0054] R₂₉, R₃₀, and R₃₁ are independently hydrogen, alkyl, alkenyl,alkynyl, monocyclic aryl or monocyclic heteroaryl.

[0055] Briefly, therefore, the taxane derivatives are prepared byreacting a β-lactam (2) with an alkoxide having the bi-, tri- ortetracyclic taxane nucleus to form a β-amido ester intermediate. Theintermediate is then converted to the taxane derivative. β-lactam (2)has the general formula:

[0056] wherein R₁-R₅ are as previously defined. The alkoxide preferablyhas the tricyclic taxane nucleus corresponding to the general formula:

[0057] wherein M and R₁₅-R₂₇ are as previously defined. Most preferably,the alkoxide has the tetracyclic taxane nucleus corresponding toalkoxide (3) wherein R₂₂ and R₂₃ together form an oxetane ring.

[0058] Other objects and features of this invention will be in partapparent and in part pointed out hereinafter.

DETAILED DESCRIPTION

[0059] As used herein “Ar” means aryl; “Ph” means phenyl; “Ac” meansacetyl; “Et” means ethyl; “R” means alkyl unless otherwise defined; “Bu”means butyl; “Pr” means propyl; “TES” means triethylsilyl; “TMS” meanstrimethylsilyl; “TPAP” means tetrapropylammonium perruthenate; “DMAP”means p-dimethylamino pyridine; “DMF” means dimethylformamide; “LDA”means lithium diisopropylamide; “LAH” means lithium aluminum hydride;“Red-Al” means sodium bis(2-methoxyethoxy) aluminum hydride; “AIBN”means azo-(bis)-isobutyronitrile “10-DAB” means 10-desacetylbaccatinIII; protected hydroxy means —OR wherein R is a hydroxy protectinggroup; sulfhydryl protecting group” includes, but is not limited to,hemithioacetals such as 1-ethoxyethyl and methoxymethyl, thioesters, orthiocarbonates; “amine protecting group” includes, but is not limitedto, carbamates, for example, 2,2,2-trichloroethylcarbamate ortertbutylcarbamate; and “hydroxy protecting group” includes, but is notlimited to, ethers such as methyl, t-butyl, benzyl, p-methoxybenzyl,p-nitrobenzyl, allyl, trityl, methoxymethyl, methoxyethoxymethyl,ethoxyethyl, tetrahydropyranyl, tetrahydrothiopyranyl, and trialkylsilylethers such as trimethylsilyl ether, triethylsilyl ether,dimethylarylsilyl ether, triisopropylsilyl ether andt-butyldimethylsilyl ether; esters such as benzoyl, acetyl,phenylacetyl, formyl, mono-, di-, and trihaloacetyl such aschloroacetyl, dichloroacetyl, trichloroacetyl, trifluoroacetyl; andcarbonates including but not limited to alkyl carbonates having from oneto six carbon atoms such as methyl, ethyl, n-propyl, isopropyl, n-butyl,t-butyl; isobutyl, and n-pentyl; alkyl carbonates having from one to sixcarbon atoms and substituted with one or more halogen atoms such as2,2,2-trichloroethoxymethyl and 2,2,2-trichloroethyl; alkenyl carbonateshaving from two to six carbon atoms such as vinyl and allyl; cycloalkylcarbonates have from three to six carbon atoms such as cyclopropyl,cyclobutyl, cyclopentyl and cyclohexyl; and phenyl or benzyl carbonatesoptionally substituted on the ring with one or more C₁₋₅ alkoxy, ornitro. Other hydroxyl, sulfhydryl and amine protecting groups may befound in “Protective Groups in Organic Synthesis” by T. W. Greene, JohnWiley and Sons, 1981.

[0060] The alkyl groups described herein, either alone or with thevarious substituents defined hereinabove are preferably lower alkylcontaining from one to six carbon atoms in the principal chain and up to15 carbon atoms. They may be straight or branched chain and includemethyl, ethyl, propyl, isopropyl, butyl, hexyl and the like.

[0061] The alkenyl groups described herein, either alone or with thevarious substituents defined hereinabove are preferably lower alkenylcontaining from two to six carbon atoms in the principal chain and up to15 carbon atoms. They may be straight or branched chain and includeethenyl, propenyl, isopropenyl, butenyl, isobutenyl, hexenyl, and thelike.

[0062] The alkynyl groups described herein, either alone or with thevarious substituents defined hereinabove are preferably lower alkynylcontaining from two to six carbon atoms in the principal chain and up to15 carbon atoms. They may be straight or branched chain and includeethynyl, propynyl, butynyl, isobutynyl, hexynyl, and the like.

[0063] The aryl moieties described herein, either alone or with varioussubstituents, contain from 6 to 15 carbon atoms and include phenyl.Substituents include alkanoxy, protected hydroxy, halogen, alkyl, aryl,alkenyl, acyl, acyloxy, nitro, amino, amido, etc. Phenyl is the morepreferred aryl.

[0064] The heteroaryl moieties described herein, either alone or withvarious substituents, contain from 5 to 15 atoms and include, furyl,thienyl, pyridyl and the like. Substituents include alkanoxy, protectedhydroxy, halogen, alkyl, aryl, alkenyl, acyl, acyloxy, nitro, amino, andamido.

[0065] The acyloxy groups described herein contain alkyl, alkenyl,alkynyl, aryl or heteroaryl groups.

[0066] The heterosubstituents of the heterosubstituted alkyl, alkenyl,alkynyl, aryl, and heteroaryl moieties described herein, containnitrogen, oxygen, sulfur, halogens and/or one to six carbons, andinclude lower alkoxy such as methoxy, ethoxy, butoxy, halogen such aschloro or fluoro, and nitro.

[0067] The present invention is directed to a process for preparingsubstituted isoserine esters, in general, and taxol, taxotere and othertaxane derivatives which are biologically active using β-lactam (2), thestructure of which is depicted hereinbelow:

[0068] wherein R₁, R₂, R₃, R₄ and R₅ are as previously defined.

[0069] In accordance with the present invention, R₅ of β-lactam (2) ispreferably —COR₁₀ or —COOR₁₀ with R₁₀ being lower alkyl, aryl,heteroaryl (such as furyl or thienyl), or substituted phenyl, and mostpreferably phenyl, methyl, ethyl, tert-butyl, or

[0070] wherein X is Cl, Br, F, CH₃O—, or NO₂—. Preferably R₂ and R₄ arehydrogen or lower alkyl. R₃ is preferably aryl, most preferably,naphthyl, phenyl,

[0071] wherein X is as previously defined, Me is methyl and Ph isphenyl. Preferably, R₁ is selected from —OR₆, —SR₇ or —NR₈R₉ wherein R₆,R₇ and R₉, are hydroxy, sulfhydryl, and amine protecting groups,respectively, and R₈ is hydrogen, alkyl, alkenyl, alkynyl, aryl, orheteroaryl. Most preferably, R₁ is —OR₆ wherein R₆ is triethylsilyl(“TES”), 1-ethoxyethyl (“EE”) or 2,2,2-trichloroethoxymethyl.

[0072] As noted above, R₁ of β-lactam (2) may be —OR₆ with R₆ beingalkyl, acyl, ethoxyethyl (“EE”), triethylsilyl (“TES”),2,2,2-trichloroethoxymethyl, or other hydroxyl protecting group such asacetals and ethers, i.e., methoxymethyl (“MOM”), benzyloxymethyl;esters, such as acetates; carbonates, such as methyl carbonates; andalkyl and aryl silyl such as triethylsilyl, trimethylsilyl,dimethyl-t-butylsilyl, dimethylarylsilyl, dimethyl-heteroarylsilyl, andtriisopropylsilyl, and the like. A variety of protecting groups for thehydroxyl group and the synthesis thereof may be found in “ProtectiveGroups in Organic Synthesis” by T. W. Greene, John Wiley and Sons, 1981.The hydroxyl protecting group selected should be easily removed underconditions that are sufficiently mild, e.g., in 48% HF, acetonitrile,pyridine, or 0.5% HCl/water/ethanol, and/or zinc, acetic acid so as notto disturb the ester linkage or other substituents of the taxolintermediate. However, R₆ is preferably triethylsilyl, 1-ethoxyethyl or2,2,2-trichloroethoxymethyl, and most preferably triethylsilyl.

[0073] Since β-lactam (2) has several asymmetric carbons, it is known tothose skilled in the art that the compounds of the present inventionhaving asymmetric carbon atoms may exist in diastereomeric, racemic, oroptically active forms. All of these forms are contemplated within thescope of this invention. More specifically, the present inventionincludes enantiomers, diastereomers, racemic mixtures, and othermixtures thereof.

[0074] β-lactam (2) can be prepared from readily available materials, asis illustrated in schemes A and B below:

[0075] reagents: (a) triethylamine, CH₂Cl₂, 25° C., 18 h; (b) 4 equivceric ammonium nitrate, CH₃CN, −10° C., 10 min; (c) KOH, THF, H₂O, 0°C., 30 min; (d) ethyl vinyl ether, THF, toluene sulfonic acid (cat.), 0°C., 1.5 h; (e) n-butyllithium, ether, −78° C., 10 min; benzoyl chloride,−78° C., 1 h; (f) lithium diisopropyl amide, THF −78° C. to −50° C.; (g)lithium hexamethyldisilazide, THF −78° C. to 0° C.; (h) THF, −78° C. to25° C., 12 h.

[0076] The starting materials are readily available. In scheme A,α-acetoxy acetyl chloride is prepared from glycolic acid, and, in thepresence of a tertiary amine, it cyclocondenses with imines preparedfrom aldehydes and p-methoxyaniline to give1-p-methoxyphenyl-3-acyloxy-4-arylazetidin-2-ones. The p-methoxyphenylgroup can be readily removed through oxidation with ceric ammoniumnitrate, and the acyloxy group can be hydrolyzed under standardconditions familiar to those experienced in the art to provide3-hydroxy-4-arylazetidin-2-ones. The 3-hydroxyl group is protected with1-ethoxyethyl, but may be protected with variety of standard protectinggroups such as the triethylsilyl group or other trialkyl (or aryl) silylgroups. In Scheme B, ethyl-α-triethylsilyloxyacetate is readily preparedfrom glycolic acid.

[0077] The racemic β-lactams may be resolved into the pure enantiomersprior to protection by recrystallization of the corresponding2-methoxy-2-(trifluoromethyl) phenylacetic esters. However, the reactiondescribed hereinbelow in which the β-amido ester side chain is attachedhas the advantage of being highly diastereoselective, thus permittingthe use of a racemic mixture of side chain precursor.

[0078] The 3-(1-ethoxyethoxy)-4-phenylazetidin-2-one of scheme A and the3-(1-triethylsilyloxy)-4-phenylazetidin-2-one of scheme B can beconverted to β-lactam (2), by treatment with a base, preferablyn-butyllithium, and an acyl chloride, alkylchloroformate, sulfonylchloride, phosphinyl chloride or phosphoryl chloride at −78° C. orbelow.

[0079] The process of the present invention is particularly useful forthe esterification of mono- or polycyclic alkoxides represented by theformula

[0080] in which E₁, E₂ and the carbon to which they are attached definea carbocyclic and/or heterocyclic skeleton which may be mono- orpolycyclic and E₃ is hydrogen or hydrocarbon, preferably lower alkyl.Most preferably, the carbocyclic and/or heterocyclic skeleton comprisesabout 6 to 20 atoms and the hetero atoms are oxygen. The cyclic skeletonmay be hydrocarbon and/or heterosubstituted with heterosubstituentsincluding, for example, esters, ethers, amines, alcohols, protectedalcohols, carbonyl groups, halogens, oxygen, substituted oxygen orsubstituted nitrogen.

[0081] When the alkoxides have the bi-, tri- or tetracyclic taxanenucleus, the process of the present invention may advantageously be usedto prepare taxane derivatives, many of which have been found to havesignificant biological activity. As used herein, an alkoxide having thebicyclic taxane nucleus has the carbocyclic skeleton corresponding torings A and B of metal alkoxide (3):

[0082] wherein M and R₁₅-R₂₇ are as previously defined. An alkoxidehaving the tricyclic taxane nucleus has the carbocyclic skeletoncorresponding to rings A, B and C of metal alkoxide (3). An alkoxidehaving the tetracyclic taxane nucleus has carbocyclic rings A, B and Cof metal alkoxide (3) and the oxetane ring defined by R₂₂, R₂₃, and thecarbons to which they are attached.

[0083] Substituent, M, of alkoxide 3 is a metal or comprises ammonium.The metal may be a Group IA, IIA, transition (including lanthanides andactinides), IIB, IIIA IVA, VA, or VIA metal (CAS version). The ammoniumcomprising substituent is preferably tetraalkylammonium and the alkylcomponent of the tetraalkylammonium substituent is preferably C₁-C₁₀alkyl such as methyl or butyl.

[0084] Alkoxides (3) are prepared by reacting an alcohol having two tofour rings of the taxane nucleus and a C-13 hydroxyl group with anorganometallic compound, zinc chloride dimethoxyethane complex,anhydrous cadmium chloride, or a tetraalkylammonium halide such astetrabutylammonium chloride in a suitable solvent. Preferably, thealcohol is a derivative of baccatin III or 10-deacetyl baccatin IIIhaving the structure:

[0085] wherein R₁₆-R₂₀, R₂₄, and R₂₆ are as previously defined. Morepreferably, R₁₆ is protected hydroxy or —OCOR₂₉, R₁₉ is hydrogen, R₂₀ ishydrogen, halogen or protected hydroxy, R₂₄ is acetoxy, and R₂₆ isbenzoyloxy Most preferably, the alcohol is a protected baccatin III, inparticular, 7-O-triethylsilyl baccatin III (which can be obtained asdescribed by Greene, et al. in JACS 110, 5917 (1988) or by other routes)or 7,10-bis-O-triethylsilyl baccatin III.

[0086] As reported in Greene et al., 10-deacetyl baccatin III isconverted to 7-O-triethylsilyl-10-deacetyl baccatin III according to thefollowing reaction scheme:

[0087] Under what is reported to be carefully optimized conditions,10-deacetyl baccatin III is reacted with 20 equivalents of (C₂H₅)₃SiClat 23° C. under an argon atmosphere for 20 hours in the presence of 50ml of pyridine/mmol of 10-deacetyl baccatin III to provide7-triethylsilyl-10-deacetyl baccatin III (6a) as a reaction product in84-86% yield after purification. The reaction product is then acetylatedwith 5 equivalents of CH₃COCl and 25 mL of pyridine/mmol of (6a) at 0°C. under an argon atmosphere for 48 hours to provide 86% yield of7-O-triethylsilyl baccatin III (6b). Greene, et al. in JACS 110, 5917 at5918 (1988).

[0088] Alternatively, 7-triethylsilyl-10-deacetyl baccatin III (6a) canbe protected at C-10 oxygen with an acid labile hydroxyl protectinggroup. For example, treatment of (6a) with n-butyllithium in THFfollowed by triethylsilyl chloride (1.1 mol equiv.) at 0° C. gives7,10-bis-O-triethylsilyl baccatin III (6c) in 95% yield. Also, (6a) canbe converted to 7-O-triethylsilyl-10-(1-ethoxyethyl) baccatin III (6d)in 90% yield by treatment with excess ethyl vinyl ether and a catalyticamount of methane sulfonic acid. These preparations are illustrated inthe reaction scheme below.

[0089] 7-O-triethylsilyl baccatin III (6b), 7,10-bis-O-triethylsilylbaccatin III (6c), or 7-O-triethylsilyl-10-(1-ethoxyethyl) baccatin III(6d) is reacted with an organometallic compound such as n-butyllithiumin a solvent such as tetrahydrofuran (THF), to form the metal alkoxide13-O-lithium-7-O-triethylsilyl baccatin III (7b)13-O-lithium-7,10-bis-O-triethylsilyl baccatin III (7c), or13-O-lithium-7-O-triethylsilyl-10-(1-ethoxyethyl) baccatin III (7d) asshown in the following reaction scheme:

[0090] As illustrated in the following reaction scheme, a suitable metalalkoxide of the present invention such as 13-O-lithium-7-O-triethylsilylbaccatin III derivative (7b, 7c, or 7d) reacts with a β-lactam of thepresent invention to provide an intermediate (8b, 8c, or 8d) in whichthe C-7 hydroxyl group is protected with a triethylsilyl or1-ethoxyethyl group.

[0091] Intermediate compound (8b) readily converts to taxol when R₁ is—OR₆, R₂ and R₃ are hydrogen, R₄ is phenyl, R₅ is benzoyl and R₆ is ahydroxy protecting group such as triethylsilyl. Intermediate compound(8c) readily converts to taxotere when R₁ is —OR₆, R₂ and R₃ arehydrogen, R₄ is phenyl, R₅ is tertbutoxycarbonyl and R₆ is a hydroxyprotecting group such as triethylsilyl. Intermediate compound (8d)readily converts to 10-deacetyl taxol when R₁ is —OR₆, R₂ and R₃ arehydrogen, R₄ is phenyl, R₅ is benzoyl, and R₆ is a hydroxy protectinggroup such as triethylsilyl. Intermediate compounds (8b, 8c and 8d) maybe converted to the indicated compounds by hydrolyzing the triethylsilyland 1-ethoxyethyl groups under mild conditions so as not to disturb theester linkage or the taxane derivative substituents.

[0092] Both the conversion of the alcohol to the metal alkoxide and theultimate synthesis of the taxol can take place in the same reactionvessel. Preferably, the β-lactam is added to the reaction vessel afterformation therein of the metal alkoxide.

[0093] The organometallic compound n-butyllithium is preferably used toconvert the alcohol to the corresponding metal alkoxide, but othersources of metallic substituent such as lithium diisopropyl amide, otherlithium or magnesium amides, ethylmagnesium bromide, methylmagnesiumbromide, other organolithium compounds, other organomagnesium compounds,organosodium, organotitanium, organozirconium, organozinc, organocadmiumor organopotassium or the corresponding amides may also be used.Organometallic compounds are readily available, or may be prepared byavailable methods including reduction of organic halides with metal.Lower alkyl halides are preferred. For example, butyl bromide can bereacted with lithium metal in diethyl ether to give a solution ofn-butyllithium in the following manner:

[0094] Alternatively, the lithium alkoxide may be induced to undergoexchange with metal halides to form alkoxides of aluminum, boron,cerium, calcium, zirconium or zinc.

[0095] Although THF is the preferred solvent for the reaction mixture,other ethereal solvents, such as dimethoxyethane, or aromatic solventsmay also be suitable. Certain solvents, including some halogenatedsolvents and some straight-chain hydrocarbons in which the reactants aretoo poorly soluble, are not suitable. Other solvents are not appropriatefor other reasons. For example, esters are not appropriate for use withcertain organometallic compounds such as n-butyllithium due toincompatibility therewith.

[0096] Although the reaction scheme disclosed herein is directed to thesynthesis of certain taxol derivatives, it can be used withmodifications in either the β-lactam or the tetracyclic metal alkoxide.Therefore, alkoxides other than 13-O-lithium-7-O-triethylsilyl baccatinIII may be used to form a taxol or other taxanes according to the methodof this invention. The β-lactam and the tetracyclic alkoxide can bederived from natural or unnatural sources, to prepare other synthetictaxols, taxol derivatives, 10-deacetyltaxols, and the enantiomers anddiastereomers thereof contemplated within the present invention.

[0097] The process of the invention also has the important advantage ofbeing highly diastereoselective. Therefore racemic mixtures of the sidechain precursors may be used. Substantial cost savings may be realizedbecause there is no need to resolve racemic β-lactams into their pureenantiomers. Additional cost savings may be realized because less sidechain precursor, e.g., 60-70% less, is required relative to priorprocesses.

[0098] The water solubility of compounds of formula (3) may be improvedby modification of the C2′ and/or C7 substituents. For instance, watersolubility may be increased if R₁ is —OR₆ and R₂₀ is —OR₂₈, and R₆ andR₂₈ are independently hydrogen or —COGCOR¹ wherein

[0099] G is ethylene, propylene, —CH═CH—, 1,2-cyclohexane, or1,2-phenylene,

[0100] R¹=OH base, NR²R³, OR³, SR³, OCH₂CONR⁴R⁵, OH

[0101] R²=hydrogen, methyl

[0102] R³=(CH₂)_(n)NR⁶R⁷; (CH₂)_(n)N⁹ R⁶R⁷R⁸X⁰

[0103] n=1 to 3

[0104] R⁴=hydrogen, lower alkyl containing 1 to 4 carbons

[0105] R⁵=hydrogen, lower alkyl containing 1 to 4 carbons, benzyl,hydroxyethyl, CH₂CO₂H, dimethylaminoethyl

[0106] R⁶R⁷=lower alkyl containing 1 or 2 carbons, benzyl or R⁶ and

[0107] R⁷ together with the nitrogen atom of NR⁶R⁷ form the followingrings

[0108] R⁸=lower alkyl containing 1 or 2 carbons, benzyl

[0109] X⁰=halide

[0110] base=NH₃, (HOC₂H₄)₃N, N(CH₃)₃, CH₃N(C₂H₄OH)₂, NH₂(CH₂)₆NH₂,N-methylglucamine, NaOH, KOH.

[0111] The preparation of compounds in which X₁ or X₂ is —COGCOR¹ is setforth in Hangwitz U.S. Pat. No. 4,942,184 which is incorporated hereinby reference.

[0112] Alternatively, water solubility may be increased when R₁ is —OR₆and R₆ is a radical having the formula —COCX═CHX or —COX—CHX—CHX—SO₂O-Mwherein X is hydrogen, alkyl or aryl and M is hydrogen, alkaline metalor an ammonio group as described in Kingston et al., U.S. Pat. No.5,059,699 (incorporated herein by reference).

[0113] Taxanes having alternative C9 keto substituent may be prepared byselectively reduction to yield the corresponding C9 β-hydroxyderivative. The reducing agent is preferably a borohydride and, mostpreferably, tetrabutyl-ammoniumborohydride (Bu₄NBH₄) ortriacetoxyborohydride.

[0114] As illustrated in Reaction Scheme 1, the reaction of baccatin IIIwith Bu₄NBH₄ in methylene chloride yields 9-desoxo-9β-hydroxybaccatinIII 5. After the C7 hydroxy group is protected with the triethylsilylprotecting group, for example, a suitable side chain may be attached to7-protected-9β-hydroxy derivative 6 as elsewhere described herein.Removal of the remaining protecting groups thus yields 9β-hydroxy-desoxotaxol or other 9β-hydroxytetracylic taxane having a C13 side chain.

[0115] Alternatively, the C13 hydroxy group of 7-protected-9β-hydroxyderivative 6 may be protected with trimethylsilyl or other protectinggroup which can be selectively removed relative to the C7 hydroxyprotecting group as illustrated in Reaction Scheme 2, to enable furtherselective manipulation of the 1 various substituents of the taxane. Forexample, reaction of 7,13-protected-9β-hydroxy derivative 7 with KHcauses the acetate group to migrate from C10 to C9 and the hydroxy groupto migrate from C9 to C10, thereby yielding 10-desacetyl derivative 8.Protection of the C10 hydroxy group of 10-desacetyl derivative 8 withtriethylsilyl yields derivative 9. Selective removal of the C13 hydroxyprotecting group from derivative 9 yields derivative 10 to which asuitable side chain may be attached as described above.

[0116] As shown in Reaction Scheme 3,10-oxo derivative 11 can beprovided by oxidation of 10-desacetyl derivative 8. Thereafter, the C13hydroxy protecting group can be selectively removed followed byattachment of a side chain as described above to yield9-acetoxy-10-oxo-taxol or other 9-acetoxy-10-oxotetracylic taxaneshaving a C13 side chain. Alternatively, the C9 acetate group can beselectively removed by reduction of 10-oxo derivative 11 with a reducingagent such as samarium diiodide to yield 9-desoxo-10-oxo derivative 12from which the C1-3 hydroxy protecting group can be selectively removedfollowed by attachment of a side chain as described above to yield9-desoxo-10-oxo-taxol or other 9-desoxo-10-oxotetracylic taxanes havinga C13 side chain.

[0117] Reaction Scheme 4 illustrates a series of reactions in which10-DAB is used as the starting material. Reduction of 10-DAB yieldspentaol 13, the C7 and C10 hydroxyl groups of which can be selectivelyprotected with the triethylsilyl or another protecting group to producetriol 14. A C13 side chain can be attached to triol 14 as describedabove or, alternatively, after further modification of the tetracylicsubstituents.

[0118] Taxanes having C9 and/or C10 acyloxy substituents other thanacetate can be prepared using 10-DAB as a starting material asillustrated in Reaction Scheme 5. Reaction of 10-DAB with triethylsilylchloride in pyridine yields 7-protected 10-DAB 15. The C10 hydroxysubstituent of 7-protected 10-DAB 15 may then be readily acylated withany standard acylating agent to yield derivative 16 having a new C10acyloxy substituent. Selective reduction of the C9 keto substituent ofderivative 16 yields 9β-hydroxy derivative 17 to which a C13 side chainmay be attached. Alternatively, the C10 and C9 groups can be caused tomigrate as set forth in Reaction Scheme 2, above.

[0119] Taxanes having alternative C2 and/or C4 esters can be preparedusing baccatin III and 10-DAB as starting materials. The C2 and/or C4esters of baccatin III and 10-DAB can be selectively reduced to thecorresponding alcohol(s) using reducing agents such as LAH or Red-Al,and new esters can thereafter be substituted using standard acylatingagents such as anhydrides and acid chlorides in combination with anamine such as pyridine, triethylamine, DMAP, or diisopropyl ethyl amine.Alternatively, the C2 and/or C4 alcohols may be converted to new C2and/or C4 esters through formation of the corresponding alkoxide bytreatment of the alcohol with a suitable base such as LDA followed by anacylating agent such as an acid chloride.

[0120] Baccatin III and 10-DAB analogs having different substituents atC2 and/or C4 can be prepared as set forth in Reaction Schemes 6-10. Tosimplify the description, 10-DAB is used as the starting material. Itshould be understood, however, that baccatin III derivatives or analogsmay be produced using the same series of reactions (except for theprotection of the C10 hydroxy group) by simply replacing 10-DAB withbaccatin III as the starting material. 9-desoxo derivatives of thebaccatin III and 10-DAB analogs having different substituents at C2and/or C4 can then be prepared by reducing the C9 keto substituent ofthese analogs and carrying out the other reactions described above.

[0121] In Reaction Scheme 6, protected 10-DAB 3 is converted to thetriol 18 with lithium aluminum hydride. Triol 18 is then converted tothe corresponding C4 ester using Cl₂CO in pyridine followed by anucleophilic agent (e.g., Grignard reagents or alkyllithium reagents).

[0122] Alternatively, deprotonation of triol 18 with LDA followed byintroduction of an acid chloride selectively gives the C4 ester. Forexample, when acetyl chloride was used, triol 18 was converted to 1,2diol 4 as set forth in Reaction Scheme 7.

[0123] As set forth in Reaction Scheme 9, other C4 substituents can beprovided by reacting carbonate 19 with an acid chloride and a tertiaryamine to yield carbonate 22 which is then reacted with alkyllithiums orGrignard reagents to provide 10-DAB derivatives having new substituentsat C2.

[0124] Alternatively, baccatin III may be used as a starting materialand reacted as shown in Reaction Scheme 10. After being protected at C7and C13, baccatin III is reduced with LAH to produce 1,2,4,10 tetraol24. Tetraol 24 is converted to carbonate 25 using Cl₂CO and pyridine,and carbonate 25 is acylated at C10 with an acid chloride and pyridineto produce carbonate 26 (as shown) or with acetic anhydride and pyridine(not shown). Acetylation of carbonate 26 under vigorous standardconditions provides carbonate 27 which is then reacted with alkyllithiums to provide the baccatin III derivatives having new substituentsat C2 and C10.

[0125] 9-desoxo-10-desacetoxy derivatives of baccatin III and9-desoxo-10-desoxy derivatives of 10-DAB may be prepared by reactingbaccatin III or 10-DAB (or their derivatives) with samarium diiodide andthereafter reducing the C9 keto substituent as otherwise describedherein. Reaction between the tetracyclic taxane having a C10 leavinggroup and samarium diiodide may be carried out at 0° C. in a solventsuch as tetrahydrofuran. Advantageously, the samarium diiodideselectively abstracts the C10 leaving group; C13 side chains and othersubstituents on the tetracyclic nucleus remain undisturbed.

[0126] C7 dihydro and other C7 substituted taxanes can be prepared asset forth in Reaction Schemes 11 and 12.

[0127] As shown in Reaction Scheme 12, Baccatin III may be convertedinto 7-fluoro baccatin III by treatment with diethylaminosulfurtrifluoride (DAST) at room temperature in THF solution. Other baccatinderivatives with a free C7 hydroxyl group behave similarly.Alternatively, 7-chloro baccatin III can be prepared by treatment ofbaccatin III with methane sulfonyl chloride and triethylamine inmethylene chloride solution containing an excess of triethylaminehydrochloride.

[0128] A wide variety of tricyclic taxanes are naturally occurring, andthrough manipulations analogous to those described herein, anappropriate side chain can be attached to the C13 oxygen of thesesubstances. Alternatively, as shown in Reaction Scheme 13,7-O-triethylsilyl baccatin III can be converted to a tricyclic taxanethrough the action of trimethyloxonium tetrafluoroborate in methylenechloride solution. The product diol then reacts with lead tetraacetateto provide the corresponding C4 ketone. This ketone can be reduced tothe alcohol with a hydride reducing agent such as sodium borohydride andsubsequent acetylation produces the C4, C5 diacetate.

[0129] The following examples are provided to more fully illustrate theinvention.

EXAMPLE 1

[0130] Taxol Using Zinc Alkoxide

[0131] To a solution of 7-triethylsilyl baccatin III (100 mg, 0.143mmol)) in 1 mL of THF at −45° C. was added dropwise 0.286 mL of a 0.5 Msolution of potassium hexamethyldisilazide in toluene. After 15 min, 32mg (0.143 mmol) of zinc chloride dimethoxyethane complex was added.After an additional 1 h at −45° C., the mixture was warmed to 0° C. anda solution of(+)-cis-1-benzoyl-3-triethylsilyloxy-4-phenylazetidin-2-one (82 mg,0.215 mmol) in 1 mL of THF was added dropwise to the mixture. Thesolution was stirred at 0° C. for 3 h before 1 mL of a 10% solution ofAcOH in THF was added. The mixture was partitioned between saturatedaqueous NaHCO₃ and 60/40 ethyl acetate/hexane. Evaporation of theorganic layer gave a residue which was purified by flash chromatographyfollowed by recrystallization to give 139 mg (90%) of(2′R,3′S)-2′,7-(bis)triethylsilyl taxol.

EXAMPLE 2

[0132] Taxol Using Cadmium Alkoxide

[0133] To a solution of 7-triethylsilyl baccatin III (100 mg, 0.143mmol)) in 1 mL of THF at −45° C. was added dropwise 0.286 mL of a 0.5Msolution of potassium hexamethyldisilazide in toluene. After 15 min, 26mg (0.143 mmol) of anhydrous cadmium chloride was added. After anadditional 1 h at −45° C., the mixture is warmed to 0° C. and a solutionof (+)-cis-1-benzoyl-3-triethylsilyloxy-4-phenylazetidin-2-one (82 mg,0.215 mmol) in 1 mL of THF was added dropwise to the mixture. Thesolution was stirred at 0° C. for 3 h before 1 mL of a 10% solution ofAcOH in THF is added. The mixture was partitioned between saturatedaqueous NaHCO₃ and 60/40 ethyl acetate/hexane. Evaporation of theorganic layer gave a residue which was purified by flash chromatographyfollowed by recrystallization to give 131 mg (85%) of(2′R,3′S)-2′,7-(bis)triethylsilyl taxol.

EXAMPLE 3

[0134] Taxol Using Tetrabutylammonium Alkoxide

[0135] To a solution of 7-triethylsilyl baccatin III (100 mg, 0.143mmol)) in 1 mL of THF at −45° C. was added dropwise 0.286 mL of a 0.5 Msolution of potassium hexamethyldisilazide in toluene. After 15 min, asolution of 16 mg (0.143 mmol) of anhydrous tetramethylammonium chloridein 0.5 mL of THF was added. After an additional 1 h at −45° C., themixture was warmed to 0° C. and a solution of(+)-cis-1-benzoyl-3-triethylsilyloxy-4-phenylazetidin-2-one (82 mg,0.215 mmol) in 1 mL of THF was added dropwise to the mixture. Thesolution was stirred at 0° C. for 3 h before 1 mL of a 10% solution ofAcOH in THF was added. The mixture was partitioned between saturatedaqueous NaHCO₃ and 60/40 ethyl acetate/hexane. Evaporation of theorganic layer gave a residue which was purified by flash chromatographyfollowed by recrystallization to give 134 mg (87%) of(2′R,3′S)-2′,7-(bis)triethylsilyl taxol.

1. An ammonium alkoxide having the formula

wherein M is tetraalkylammonium; E₁ and E₂ and the carbon to which theyare attached comprise a carbocyclic or heterocyclic skeleton containingabout 6 to 20 ring atoms, the hetero atoms being oxygen; and E₃ ishydrogen or a hydrocarbon.
 2. The alkoxide of claim 1 wherein E₃ ishydrogen.
 3. The alkoxide of claim 1 wherein M is tetramethylammonium.4. The alkoxide of claim 1 wherein M is tetrabutylammonium.
 5. Thealkoxide of claim 1 wherein E₃ is hydrogen and M is tetramethylammonium.6. The alkoxide of claim 1 wherein E₃ is hydrogen and M istetrabutylammonium.
 7. The alkoxide of claim 1 wherein E₁ and E₂ and thecarbon atom to which they are attached comprise the carbocyclic skeletoncorresponding to rings A, B and C of structure


8. The alkoxide of claim 1 wherein E₁ and E₂ and the carbon atom towhich they are attached comprise the carbocyclic skeleton correspondingto rings A, B, C and the oxetane ring of structure